Electric assist steering systems are well known in the art. Electric assist steering systems that utilize a rack and pinion gear set provide assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. An electronic control system includes a torque sensor and a vehicle speed sensor. A computer receives the output signals provided by both sensors. The computer controls the amount of the assistance provided by the motor dependent upon the applied steering torque and the sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Re. Pat. No. 32,222,), assigned to TRW Inc., includes a D.C. electric assist motor driven through an H-bridge arrangement. The motor includes a rotatable armature encircling a steering member which has a thread convolution portion and a portion having straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball-nut drive arrangement drivably coupled to the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column to sense driver applied input torque to the steering wheel. The torque sensing device uses a Hall-effect sensor for sensing relative rotation between the input and output shafts across a torsion bar. An electronic control unit ("ECU") monitors the signal from the torque sensing device and controls the electric assist motor in response thereto. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The ECU controls current through the electric assist motor in response to both the sensed vehicle speed and the sensed applied steering torque. The ECU decreases steering assist as vehicle speed increases. This is commonly referred to in the art as speed proportional steering.
U.S. Pat. No. 4,660,671 discloses an electric controlled steering system that is based on the Drutchas steering gear. In the '671 arrangement, the D.C. motor is axially spaced from the ball-nut and is operatively connected thereto through a connection tube. The electronic controls includes a plurality of diagnostic features that monitor the operation of the steering system. If an error in the operation of the electric assist steering system is detected, the assist system is disabled and steering reverts to an unassisted mode.
U.S. Pat. No. 4,794,997 to North, assigned to TRW Cam Gears Limited, discloses an electric assist steering system having an electric motor operatively connected to the rack through a ball nut. A vehicle speed sensor and an applied steering torque sensor are operatively connected to an ECU. The ECU controls electric current through the motor as a function of both applied steering torque and sensed vehicle speed. The current is controlled by controlling the percentage of the pulse-width-modulated ("PWM") signal applied to the motor. As the PWM increases, assist increases. The ECU or computer is preprogrammed with discrete control curves that provide steering assist values (PWM values), also referred to as torque-out values, as a function of applied steering torque, also referred to as torque-in values, for a plurality of predetermined discrete vehicle speed values. Each vehicle speed value has an associated torque-in vs. torque-out control curve.
FIG. 3 of the '997 patent shows torque-in vs. torque-out control curves stored in the ECU. There is a torque-in vs. torque-out curve used for low speed vehicle maneuvering such as vehicle parking. Also, there is a torque-in vs. torque-out curve used for high speed maneuvering. Each of these control curves permits maximum assist should the applied steering torque reach an associated value. For vehicle speeds between the minimum speed curve and the maximum speed curve, a plurality of discrete curves are provided. The other discrete vehicle speed curves are all between the low and high speed curves. From these torque-in vs. torque-out curves, it can be seen that assist decreases as vehicle speed increases. The transition from one level of assist to another level of assist for the different vehicle speeds occurs in steps or jumps. Changes in assist level in this type of system can be felt by the vehicle operator when vehicle speed changes occur during a steering maneuver.
It is desirable to use a variable reluctance motor to provide electric assist for steering. One arrangement is disclosed in U.S. Pat. No. 5,257,828 to Miller et al. that uses a variable reluctance motor in an electric assist steering system. In accordance with the '828 patent, current to the motor, referred to as a current command signal, is functionally related to applied steering torque, vehicle speed, motor rotor position, and speed of the electric assist motor.
One concern with using a variable reluctance motor to provide steering assist is the amount of acoustic noise produced by the motor during energization. Current profile mapping tables are stored in memory. These current maps include current values vs. rotor position values. The current maps are designed to provide smooth motor operation, i.e., reduce motor torque ripple during motor operation.
The amount of noise and ripple occurring upon energization of the electric assist motor is functionally related to the number of data values stored in the mapping table. The larger the "space" between motor position vs. current values in the table, the more noise that occurs upon motor energization. To store enough values in a current map to ensure a quiet motor operation, requires a substantial amount of memory. Consideration of system size and expense dictates that a current mapping table be limited to a finite size not large enough to provide quiet motor operation. Because of a limited amount of memory in a system, a current value is selected from the look-up table corresponding to the closest motor position value in the table. This type of control results in an increase in audible motor noise.
In a four pole, variable reluctance electric motor, it has been found that audible noise is present during energization of the motor using a current mapping table having stored current values as a function of motor positions where the motor position increments are as small as every 0.5 electrical degrees (or every 0.083 mechanical degrees). When a current table has stored current values for discrete motor positions, only step changes in motor current can occur. These step changes are a major contributor to audible noise in the motor.